Description :

Vibration isolation is concerned with the development of an interface between vibration source and vibration sensitive equipment, which attenuates the vibration transmission above the corner frequency of the isolation system. As an example, a precision payload (such as a telescope) must be isolated from the jitter introduced by the reaction wheel assembly of the attitude control system of a spacecraft. On the other hand, the isolation system must allow the low frequency attitude control torque to be transmitted to the spacecraft.

Any passive isolation system consists of one or several stages of springs and dampers introduced in the vibration propagation path; their parameters are adjusted to achieve a desired corner frequency and a reasonable compromise between the amplification at the resonance and the high frequency attenuation. The passive damping is necessary to limit the amplification at resonance, but it tends to reduce the high frequency attenuation of the isolation system.

Active vibration isolation aims at improving the performance of the vibration isolation by including a force-generating element in the isolation interface, a sensor at the receiving end of the transmission path, and a feedback control law connecting them.
In order to fully isolate two rigid bodies with respect to each other, we need six of such active isolators judiciously placed, which are, in our design, controlled in a decentralized manner. A six-axis vibration isolator based on a Stewart platform was designed and manufactured for this purpose. Such a design exhibits high robustness properties as well as performances mainly independent of the payload used. This system was tested in the frame of the 33rd ESA parabolic flight campaign in September 2002. The test campaign was successful, the results being characterised by a –20dB decay rate over a frequency range of about 100Hz. Various possibilities for improving the performances were also identified.

A new isolator has thus been designed and manufactured in order to be simpler, more robust and more performing. At the same occasion, a new shaking table has been designed and a new experimental procedure has been developed in order to improve the quality of the measurements. The results, obtained during the 38th ESA parabolic flight campaign in October 2004, show a substantial improvement on the transmissibility that exhibits an impressive –40dB decay rate over a frequency range of 400Hz. Furthermore, a prototype leg close to this design has successfully undergone vibration qualification tests.

For more details about this research project, please visit our website:
http://www.ulb.ac.be/scmero/isolation.html